Today's traditional cruise controls maintain a constant reference speed which is usually set by the driver. The value of the reference speed is only altered when the driver him/herself adjusts it while driving. The reference speed is then passed to a control system which controls the vehicle in such a way that, if possible, the vehicle maintains the reference speed. If the vehicle is equipped with an automatic gear change system, its gears change so that it can maintain desired speed.
When cruise control is used in hilly terrain, the cruise control system will try to maintain set speed both uphill and downhill. This sometimes results in the vehicle accelerating over the crest of a hill and potentially into a subsequent downgrade on which it has then to be braked to avoid exceeding the set speed, which is a fuel-expensive way of running the vehicle. Even in cases where it does not accelerate over the crest, the vehicle may of course need to be braked downhill to avoid exceeding the set speed.
To reduce fuel use on undulating roads in particular, economical cruise controls such as Scania's Ecocruise® have been developed. The cruise control tries to estimate the vehicle's current speed and also has knowledge of the historical running resistance. The economical cruise control may also be provided with map data with topographical information. The vehicle is then located on the map, e.g. by means of GPS, and the running resistance along the route ahead is estimated. The vehicle's reference speed may thus be optimised for different types of road in order to save fuel.
An example of a cruise control which uses topographical information is described in the document entitled “Explicit use of road topography for model predictive cruise control in heavy trucks” by Erik Hellström, ISBN: LiTH-ISY-EX-05/3660-SE. The cruise control here is by real-time optimisation, and a cost function is used to define the optimisation criteria. A large number of different solutions are calculated and evaluated, and the solution resulting in least cost is employed. The considerable number of calculations involved makes it necessary to use for the purpose a processor with a suitably large capacity.
In other forms of cruise control the number of possible solutions is reduced by choosing instead to iterate from one solution along the vehicle's intended itinerary. However, the topography of the carriageway, the weight of the vehicle and the performance of the engine may result in differences in demand for processor capacity for determining the reference speed. More calculations are required in the case, for example, of a heavily laden truck with medium-high engine power travelling on an undulating road than in that of a lightly laden truck with a high engine power travelling on a relatively level road. This is because the truck in the first case is likely to accelerate on each downgrade and slow down on each upgrade, whereas in the second case the truck will run quite evenly.
The processor incorporated in the system will therefore be subject to relatively large demands, since the load upon the processor may vary greatly from situation to situation. For example, its capacity needs to be large enough to cope quickly with situations where a large number of calculations have to be done. It therefore needs to be dimensioned to cope with such situations despite the fact that they occur during only a limited proportion of the processor time used.
The object of the present invention is to propose an improved system for controlling a vehicle's speed so that the amount of fuel used can be minimised and, in particular, in such a way that the load upon the processor becomes lighter and more even. A further object of the invention is to propose a simplified cruise control which has predictable behaviour compared with other economical cruise controls.